Surface enhanced raman spectroscopy using shaped gold nanoparticles

ABSTRACT

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates is presented for SAMs onto which gold nanoparticles of various shapes have been electrostatically immobilized. SERS spectra of 4-MBA SAMs are enhanced in the presence of immobilized gold nanocrystals by a factor of 10 7 -10 9  relative to 4-MBA in solution. Large enhancement factors are a likely result of plasmon coupling between the nanoparticles (localized surface plasmon) and the smooth gold substrate (surface plasmon polariton), creating large localized electromagnetic fields at their interface, where 4-MBA molecules reside in this sandwich architecture. Moreover, enhancement factors depend on nanoparticle shape, and vary by a factor of 10 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. Ser. No. 11/721,554 filed Jul.7, 2008, which is a U.S. National Phase of PCT/US2005/044963 filed Dec.13, 2005, which claims the benefit of U.S. Application No. 60/635,704,filed Dec. 13, 2004, and U.S. Application No. 60/648,920, filed Feb. 1,2005, all of which are hereby incorporated herein by reference in theirentireties.

ACKNOWLEDGEMENT

This invention was made with government support under Grant CHE-0336350awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

Bulk solution synthetic methods often produce nanocrystals of multiplesizes and shapes, and hence there is relatively low yield of the desiredsize and shape. Murphy, C. J. Science 2002, 298, 2139-2141. Althoughcolloid chemists have achieved excellent control over particle size forseveral metallic and semiconductor systems, there has been limitedsuccess in gaining control over the shape of the nanocrystals. Schmid,G.; Ed. Clusters and Colloids. From Theory to Applications; VCH: NewYork, 1994. Watzky, M. A.; Finke, R. G., J. Am. Chem. Soc. 1997, 119,10382-10400. Jana, N. R.; Peng, X., J. Am. Chem. Soc. 2003, 125,14280-14281. Controlling size, shape, and structural architecture of thenanocrystals requires manipulation of the kinetic and thermodynamicparameters of the systems via utilization of various additives, lightand thermal energies, and their various combinations. Ahmadi, T. S.;Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A., Science 1996,272, 1924-1925. Pileni, M. P.; Ninham, B. W.; Gulik-Krzywicki, T.;Tanori, J.; Lisiecki, I.; Filankembo, A., Adv. Mater. 1999, 11,1358-1362. Li, M.; Schnablegger, H.; Mann, S, Nature 1999, 402, 393-395.Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C.A., Nature 2003, 425, 487-490. Sun, Y.; Xia, Y. Science 2002, 298,2176-2179. Sun, Y.; Xia, Y., Adv. Mater. 2002, 14, 833-837. Sun, Y.;Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955-960.

Therefore, there remains a need for methods and compositions thatovercome these deficiencies and that effectively provide shapednanoparticles.

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical toolfor determining chemical information for molecules on metallicsubstrates. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. In general,there are two traditional operational mechanism to describe the overallSERS effect, electromagnetic (EM) and chemical (CHEM) enhancementmechanisms. EM enhancement is enhancement of the local electromagneticfield incident on an adsorbed molecule at a metallic surface. CHEMenhancement results from electronic resonance/charge transfer between amolecule and a metal surface, which leads to an increase thepolarizability of the molecule. Otto, A.; Mrozek, I.; Pettenkofer, C.Surf Sci. 1990, 238, 192. Schultz, S. G.; Janik-Czachor, M.; Van Duyne,R. P. Surf Sci. 1984, 104, 419. Since the introduction of the SERSphenomenon on roughened silver electrodes, much attention has turned toSERS on spherical colloidal substrates of either gold or silver.Jeanmaire, D. L.; Van Duyne, R. P., J. Electroanal. Chem. 1977, 84,1-20. Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99,5215-5217. Nie, S. M.; Emery, S. R. Science 1997, 275, 1102-1106. Krug,J. T.; Wang, G. D.; Emory, S. R.; Nie, S. M., J. Am. Chem. Soc., 1999,121, 9208-9214. Freeman, R. G.; Bright, R. M.; Hommer, M. B.; Natan, M.J., J. Raman Spectrosc. 1999, 30, 733-738. Jensen, T. R.; Malinsky, M.D.; Haynes, C. L.; Van Duyne, R. P., J. Phys. Chem. B 2000, 104,10549-10556. Kneipp, K.; Kneipp, H.; Deinum, G.; Itzkan, I.; Dasari, R.R.; Feld, M. S. Appl. Spectrosc. 1998, 52, 175-178. Colloidalnanoparticles are of interest as SERS substrates not only because theyare strong light scatterers, but because of their tunable opticalproperties which depend on nanoparticle size, shape, and aggregationstate. El-Sayed, M. A., Acc. Chem. Res. 2001, 34, 257-264. Kelly, K. L.;Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phzys. Chem. B 2003, 107,668-677.

Spheroidal or rod-shaped nanoparticles are of significant interest asSERS substrates because of their tunable longitudinal plasmon bands andthe “lightning rod” effect on surface enhancement. Schatz, G. C., Acc.Chem. Res. 1984, 17, 370-376. Gersten, J. I., J. Chem. Phys. 1980, 72,5779-5780. While electric field enhancement is observed for 10-200 nMmetallic particles, even greater local field enhancements are observedat sharp surface features, for example, at the tips of needle-shapednanorods where the curvature radius is much smaller than the size of thenanoparticle. Gersten, J. I. J. Chem. Phys. 1980, 72, 5779-5780. Thisphenomenon is known as the lightning rod effect. Despite the desirablecharacteristics of metallic nanorods and nanowires as SERS substrates,only a few reports exist for SERS on rod- or wire-shaped nanoparticles.Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y,Yang, P. Nano Lett. 2003, 3, 1229-1323. Jeong, D. H.; Zhang, Y. X.;Moskovits, M., J. Phys. Chem. B 2004, 108, 12724-12728. Yao, J. L.; Pan,G. P.; Xue, K. H.; Wu, D. Y.; Ren, B.; Sun, D. M.; Tang, J.; Xu, X.;Tian, Z. Q. Pure Appl. Chem. 2000, 72, 221-228. Nikoobakht et al. haveexamined the use of unaggregated and aggregated gold nanorods as SERSsubstrates using pyridine and 4-aminothiophenol analytes. Nikoobakht, B.Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Nikoobakht,B., El-Sayed, M. A., J. Phys. Chem. A 2003, 107, 3372-3378. For SERS onunaggregated nanorods, the excitation wavelength was 1064 nm, farremoved from the nanorod absorption bands (˜520 nm and 700 nm) where theEM enhancement mechanism is thought to be inoperative. Nikoobakht, B.Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Despite theoff-resonance condition, appreciable SERS intensity was observed withsurface enhancement factors (EF) of 104 for pyridine. The authorsattributed the enhancement to a chemical (CHEM) enhancement mechanism ofstrongly adsorbed pyridine on the Au{110} surface of these nanorods.However, no reports have been made for SERS on nanorods where the Ramanexcitation occurs at a wavelength that overlaps with nanorod plasmonresonance, a condition where the EM enhancement mechanism should beoperative.

Large enhancement factors and even single molecule SERS have beenreported for molecules at junctions between aggregated nanoparticles.Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L., J. Phys. Chem. B 2003,107, 9964-9972. Xu, H. X.; Bjerneld, E. J.; Kall, M.; Borjesson, L.Phys. Rev. Lett. 1999, 83, 4357-4360. Michaels, A. M.; Jiang, J.; Brus,L., J. Phys. Chem. B 2000, 104, 11965-11971. This is a result oflocalized surface plasmon (LSP) coupling between nanoparticles andenhanced electromagnetic field intensity localized at nanoparticlejunctions. Michaels, A. M.; Jiang, J.; Brus, L., J. Phys. Chem. B 2000,104, 11965-11971. Vidal, F. J. G-.; Pendry, J. B. Phys. Rev. Lett. 1996,77, 1163-1166. Wang, D.-S.; Kerker, M. Phys. Rev. B 1981, 24, 1777-1790.Markel, V. A.; Shalaev, V. M.; Zhang, P.; Huynh, W.; Tay, L.; Haslett,T. L.; Moskovits, M. Phys. Rev. B 1999, 59, 10903-10909. Su, K.-H.; Wei,Q.-H.; Zhang, X.; Mock, J. J.; Smith, D. R.; Schultz, S, Nano Lett.2003, 3, 1087-1090. Atay, T.; Song, J-. H.; Murmikko, A. V. Nano Lett.2004, 4, 1627-1731. Fromm, D. P.; Sundaramurthy, A.; Schuck, P. J.;Kino, G.; Moemer, W. E. Nano Lett. 2004, 4, 957-961. This LSP couplingbetween aggregated gold nanorods is believed to contribute to SERSenhancement observed by El-Sayed and coworkers. Nikoobakht, B.,El-Sayed, M. A., J. Phys. Chem. A 2003, 107, 3372-3378. It is importantto note, that although it is difficult to estimate enhancement factorsfor aggregated nanoparticles, the authors stated that SERS enhancementswere always greater for aggregated gold nanorods than for aggregatedspherical nanoparticles. Nikoobakht, B., El-Sayed, M. A., J. Phys. Chem.A 2003, 107, 3372-3378. Similarly, LSP coupling between colloidalnanoparticles and the surface of planar substrates, referred to assurface plasmon polariton (SPP), has also been well documented and hasbeen reported for surface plasmon resonance (SPR) spectroscopymeasurements. Shchegrov, A. V.; Novikov, I. V.; Maradudin, A. A. Phys.Rev. Lett. 1997, 78, 4269-4272. Holland, W. R.; Hall, D. G., Phys. Rev.B 1983, 27, 7765-7768. Kume, T.; Nakagawa, N.; Yamamoto, K., Solid StateCommun. 1995, 93, 171-175. Lyon, L. A.; Musick, M. D.; Natan, M., J.Anal Chem. 1998, 70, 5177-5183. Lyon, L. A.; Pena, D. J.; Natan, M. J.,J. Phys. Chem. B 1999, 103, 5826-5831. Hutter, E.; Cha, S.; Liu, J-F.;Park, J.; Yi, J.; Fendler, J. H.; Roy, D., J. Phys. Chem. B 2001, 105,8-12. A 20-fold increase in signal is observed for biological sandwichassays where analytes are between nanoparticles and a planar surface,and LSP-SPP coupling occurs. LSP-SPP coupling has also been observedqualitatively by Zheng et al. between silver nanoparticles and surfaceplasmons of planar silver substrates. Zheng, J.; Zhou, Y.; Li, X.; Ji,Y.; Lu, T.; Gu, R Langmuir 2003, 19, 632-636. They observed greater SERSintensity for 4-aminothiophenol (4-ATP) self-assembled monolayers (SAMs)on silver when colloidal silver nanoparticles are adsorbed to the SAMthan for the 4-ATP SAM of polished and electrochemically roughenedsilver. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, R Langmuir2003, 19, 632-636.

A significant challenge in SERS on colloidal nanoparticle substrates isdetermining the number of analyte molecules sampled during theexperiment. It is essential to calculate not only the number ofnanoparticles in solution, but also the surface coverage of analytemolecules adsorbed to these nanoparticles. This is especially difficultfor nanoparticles that are synthesized using strongly adsorbed cappingagents including cetyltrimethylammonium bromide (CTAB), which may or maynot be displaced by the analyte of interest. Nikoobakht, B. Wang, J.El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. Nikoobakht, B.;El-Sayed, M. A. Langmuir 2001, 17, 6368-6374. In most reports, monolayersurface coverage on the nanocrystals is assumed, but if incorrect couldlead to errors in calculations of EF values. For SERS on self-assembledmonolayers (SAMs) on planar substrates, this problem is avoidedaltogether because there are no capping agents on these substrates andthe number of molecules sampled is well known. Ulman, A. Chem. Rev.1996, 96, 1533-1554. However, the tunability of the optical propertiesof planar SERS substrates is more difficult than solution-preparedcolloids.

Previous research has demonstrated the high yield synthesis of goldnanorods and a plethora of other shapes of nanocrystals. Jana, N. R.;Gearheart, L. Murphy, C., J. Adv. Mater. 2001, 137, 1389-1393. Jana, N.R.; Gearheart, L. Murphy, C. J., J. Phys. Chem. B 2001, 105, 4065-4067.Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414-6420. Sau, T. K.;Murphy, C. J., J. Am. Chem. Soc. 2004, 126, 8648-8649. Additionally, theimmobilization of CTAB-protected gold nanorods on carboxylate-terminatedSAMs has been studied. Gole, A.; Orendorff, C. J.; Murphy, C. J.Langmuir 2004, 20, 7117-7122. In the present invention, CTAB-cappednanoparticles of various shapes are immobilized on 4-mercaptobenzoicacid (4-MBA) monolayers. SERS spectra of 4-MBA are acquired to determinethe effect of immobilizing gold nanoparticles on SERS of 4-MBA SAMS ongold and to determine whether the nanoparticle shapes, specificallytheir optical properties and surface structure, influence SERS of 4-MBASAMs.

Therefore, there remains a need for methods and compositions thatovercome these deficiencies and that effectively provide surfaceenhanced Raman spectroscopy.

SUMMARY

In accordance with the purpose(s) of the invention, as embodied andbroadly described herein, the invention, in one aspect, relates tomethods, products, and compositions for preparing and using shapednanoparticles. In a further aspect, the invention relates to methods,products, and compositions for surface enhanced Raman spectroscopy.

In one aspect, the invention relates to a method for enhancing a Ramansignal comprising the steps of providing a sample comprising a metalsurface, an analyte adhered to the surface, and a metallic nanoparticlecoupled to the surface, wherein the nanoparticle has a plasmon resonanceband; exposing the sample to incident energy of an excitation wavelengththat overlaps with the metallic nanoparticle plasmon resonance band; anddetecting the Raman signal of the analyte.

In a further aspect, the invention relates to a method for enhancing aRaman signal comprising the steps of providing a sample comprising ametal surface, a functionalized self-assembled monolayer adhered to thesurface, wherein the self-assembled monolayer comprises an analyte, anda cetyltrimethylammonium bromide-capped metallic nanoparticle coupled tothe surface; exposing the sample to incident energy of an excitationwavelength; and detecting the Raman signal of the analyte.

In a further aspect, the invention relates to a method for enhancing aRaman signal comprising the steps of providing a sample comprising agold surface, a functionalized self-assembled monolayer adhered to thesurface, wherein the self-assembled monolayer comprises an analyte, anda cetyltrimethylammonium bromide-capped metallic nanoparticle coupled tothe surface; exposing the sample to incident energy of an excitationwavelength that overlaps with the metallic nanoparticle plasmonresonance band; and detecting the Raman signal of the analyte, whereinthe Raman signal has an enhancement factor of from about 10⁷ to about10⁹ relative to the analyte in solution.

In a further aspect, the invention relates to a composition comprising ametal surface, a functionalized self-assembled monolayer adhered to thesurface, wherein the self-assembled monolayer comprises an analyte, anda cetyltrialkylammonium halide-capped metallic nanoparticle coupled tothe surface.

In a further aspect, the invention relates to a method for preparing acetyltrimethylammonium bromide-capped gold nanoparticle comprising thesteps of providing a seed solution comprising a gold nanoparticle;providing an aqueous growth solution comprising cetyltrimethylammoniumbromide, hydrogen tetrachloroaurate, and ascorbic acid; and adding aquantity of the seed solution to the aqueous growth solution, therebyproducing a cetyltrimethylammonium bromide-capped gold nanoparticlehaving a shape comprising a cube, a block, a tetrapod, a sphere, a rod,a star, or a dogbone.

In a further aspect, the invention relates to the products produced bythe methods of the invention.

In a further aspect, the invention relates to a cetyltrimethylammoniumbromide-capped gold nanoparticle.

In a further aspect, the invention relates to a composition comprisingnanoparticulate gold and a cetyltrimethylammonium bromide residue,wherein the composition has a shape comprising a cube, a block, atetrapod, a sphere, a rod, a star, or a dogbone.

Additional advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by means ofthe elements and combinations particularly pointed out in the appendedclaims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several embodiments and togetherwith the description serve to explain the principles of the invention.

FIG. 1 shows TEM (inset SEM) images of Au nanoparticles synthesizedunder different conditions. AA increases from A to C; and seedconcentration increases from C to D. Scale bar=100 nm. See also Table 1.

FIG. 2 shows TEM images showing cubic to rod-shaped gold particlesproduced with low AA concentrations in the presence of a small quantityof silver nitrate. CTAB is increased from 1.6×10⁻² M (A), to 9.5×10⁻² M(B,C,D). Au³⁺ decreases from (B) to (C), whereas seed concentrationincreases from C to D. Scale bar=100 nm. See also Table 1.

FIG. 3 shows TEM images of branched Au nanoparticles, varying in thedimension and number of branches, prepared under various combinations of[seed]/[Au³⁺] ratio, and the concentrations of CTAB and AA. Tetra-pods(A), star-shape (B), larger tetra-pods (C), and multi-pods (D and E).See also Table 1.

FIG. 4 shows optical absorption spectra of the solutions containing Aunanocrystals of various shapes. Solutions contain a: multiple shapes; b& c: particles with hexagonal and cubic profiles, respectively. Particlesolutions correspond to the shapes given in FIGS. 1A, 1B, and 1C,respectively. d: cubic particles corresponding to that given in FIG. 2A.e: rectangular particles corresponding to that given in FIG. 2C. f:tetrapods corresponding to that given in FIG. 3A.

FIG. 5 shows UV-vis-NIR absorption spectra of (a) spheres (--), aspectratio 3.2 rods (- - - - -) aspect ratio 4.4 rods (. . . . .) aspectratio 16 rods (--), (b) dogbones (--), cubes (. . . . .) tetrapods (--),and blocks (- - - - -).

FIG. 6 shows SEM and TEM (inset) images of (a) aspect ratio 16 rods, (b)aspect ratio 3.2 rods, (c) aspect ratio 4.4 rods, (d) spheres, (e)tetrapods, (f) dogbones, (g) cubes, and (h) blocks immobilized on 4-MBASAMs.

FIG. 7 shows a scheme of the nanoparticle-SAM sandwich geometry for SERSof 4-MBA.

FIG. 8 shows Raman spectra of (a) 0.01 M 4-MBA and (b) 4-MBA SAM ongold, and SERS spectra of 4-MBA SAMs on gold with immobilized (c)spheres, (d) aspect ratio 3.2 rods, (e) aspect ratio 4.4 rods, (f)aspect ratio 16 rods, (g) cubes, (h) blocks, (i) tetrapods, and (j)dogbones. Integration times are (a) 300 s, (b) 480 s, (c) 300 s, (d) 120s, (e) 120 s, (f) 30 s, (g) 30 s, (h) 30 s, (i) 30 s, and (j) 30 s.

DETAILED DESCRIPTION

The present invention may be understood more readily by reference to thefollowing detailed description of aspects of the invention and theExamples included therein and to the Figures and their previous andfollowing description.

Before the present compounds, compositions, articles, devices, and/ormethods are disclosed and described, it is to be understood that theyare not limited to specific synthetic methods unless otherwisespecified, or to particular reagents unless otherwise specified, as suchmay, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodiments onlyand is not intended to be limiting.

A. DEFINITIONS

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, example methods andmaterials are now described.

All publications mentioned herein are incorporated herein by referenceto disclose and describe the methods and/or materials in connection withwhich the publications are cited. The publications discussed herein areprovided solely for their disclosure prior to the filing date of thepresent application. Nothing herein is to be construed as an admissionthat the present invention is not entitled to antedate such publicationby virtue of prior invention. Further, the dates of publication providedherein may be different from the actual publication dates, which mayneed to be independently confirmed.

As used in the specification and the appended claims, the singular forms“a,” “an” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a component,” “apolymer,” or “a particle” includes mixtures of two or more suchcomponents, polymers, or particles, and the like.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint. It is also understood that there are a number ofvalues disclosed herein, and that each value is also herein disclosed as“about” that particular value in addition to the value itself. Forexample, if the value “10” is disclosed, then “about 10” is alsodisclosed. It is also understood that when a value is disclosed that“less than or equal to” the value, “greater than or equal to the value”and possible ranges between values are also disclosed, as appropriatelyunderstood by the skilled artisan. For example, if the value “10” isdisclosed the “less than or equal to 10” as well as “greater than orequal to 10” is also disclosed. It is also understood that throughoutthe application, data is provided in a number of different formats andthat this data represents endpoints and starting points, and ranges forany combination of the data points.

For example, if a particular data point “10” and a particular data point15 are disclosed, it is understood that greater than, greater than orequal to, less than, less than or equal to, and equal to 10 and 15 areconsidered disclosed as well as between 10 and 15. It is also understoodthat each unit between two particular units are also disclosed. Forexample, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are alsodisclosed.

A residue of a chemical species, as used in the specification andconcluding claims, refers to the moiety that is the resulting product ofthe chemical species in a particular reaction scheme or subsequentformulation or chemical product, regardless of whether the moiety isactually obtained from the chemical species. Thus, an ethylene glycolresidue in a polyester refers to one or more —OCH₂CH₂O— units in thepolyester, regardless of whether ethylene glycol was used to prepare thepolyester. Similarly, a sebacic acid residue in a polyester refers toone or more —CO(CH₂)₈CO— moieties in the polyester, regardless ofwhether the residue is obtained by reacting sebacic acid or an esterthereof to obtain the polyester.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

Disclosed are the components to be used to prepare the compositions ofthe invention as well as the compositions themselves to be used withinthe methods disclosed herein. These and other materials are disclosedherein, and it is understood that when combinations, subsets,interactions, groups, etc. of these materials are disclosed that whilespecific reference of each various individual and collectivecombinations and permutation of these compounds may not be explicitlydisclosed, each is specifically contemplated and described herein. Forexample, if a particular compound is disclosed and discussed and anumber of modifications that can be made to a number of moleculesincluding the compounds are discussed, specifically contemplated is eachand every combination and permutation of the compound and themodifications that are possible unless specifically indicated to thecontrary. Thus, if a class of molecules A, B, and C are disclosed aswell as a class of molecules D, E, and F and an example of a combinationmolecule, A-D is disclosed, then even if each is not individuallyrecited each is individually and collectively contemplated meaningcombinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considereddisclosed. Likewise, any subset or combination of these is alsodisclosed. Thus, for example, the sub-group of A-E, B-F, and C-E wouldbe considered disclosed. This concept applies to all aspects of thisapplication including, but not limited to, steps in methods of makingand using the compositions of the invention. Thus, if there are avariety of additional steps that can be performed it is understood thateach of these additional steps can be performed with any specificembodiment or combination of embodiments of the methods of theinvention.

It is understood that the compositions disclosed herein have certainfunctions. Disclosed herein are certain structural requirements forperforming the disclosed functions, and it is understood that there area variety of structures that can perform the same function that arerelated to the disclosed structures, and that these structures willtypically achieve the same result.

B. Shaped Nanoparticles

Various aspect ratio Ag and Au nanorods, μg nanowires, and cubic Cu₂Oparticles in aqueous solution have been produced. Jana, N. R.;Gearheart, L.; Murphy, C. J., J. Phys. Chem. B 2001, 105, 4065-4067.Jana, N. R.; Gearheart, L.; Murphy, C., J. Adv. Mater. 2001, 13,1389-1393. Jana, N. R.; Gearheart, L.; Murphy, C., J. Chem. Commun.2001, 617-618. Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19,9065-9070. Gou, L.; Murphy, C. J. Nano Lett. 2003, 3, 231-234. Basedupon electron diffraction analysis and high-resolution transmissionelectron microscopy studies, the mechanism for the evolution ofcylindrical rod shapes in aqueous solution by the seeded growth methodwas investigated. Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C.J.; Mann, S. J. Mater. Chem. 2002, 12, 1765-1770. Alivisatos et al.,Peng et al., and Cheon et al. utilized high-temperature solution methodsto obtain a score of interesting shapes for semiconductor systems. Peng,X.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.;Alivisatos, A. P. Nature 2000, 404, 59-61. Manna, L.; Scher, E. C.;Alivisatos, A. P., J. Am. Chem. Soc. 2000, 122, 12700-12706. Manna, L.;Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater.2003, 2, 382-385. Peng, Z. A.; Peng, X., J. Am. Chem. Soc. 2002, 124,3343-3353. Peng, X. Adv. Mater. 2003, 15, 459-463. Lee, S.-M.; Jun,Y.-W.; Cho, S.-N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244-11245.Au particles with hexagonal (icosahedral) and pentagonal (decahedral)profiles have been synthesized by vapor deposition methods. Yacaman, M.J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J. J. Vac. Sci.Technol. B 2001, 19, 1091-1103. Yang, C. Y.; Heinemann, K.; Yacaman, M.J.; Poppa, H. Thin Solid Films 1979, 58, 163-168. Renou, A.; Gillet, M.Surf Sci. 1981, 106, 27-34. Recently, Chen et al. and Hao et al.reported the synthesis of a mixture of branched gold Au nanocrystals byusing two different colloid chemical synthetic protocols. Chen, S.;Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem.Soc. 2003, 125, 16186-16187. Hao, E.; Bailey, R. C.; Schatz, G. C.;Hupp, J. T.; Li, S, Nano Lett. 2004, 4, 327-330.

1. Preparation Methods

In one aspect, the methods of the invention involve the preparation ofAu seed particles and the subsequent addition of an appropriate quantityof the Au seed solution to the aqueous growth solutions containingdesired quantities of cetyltrimethylammonium bromide (CTAB), HAuCl₄,ascorbic acid (AA), and, optionally, a small quantity of AgNO₃.

While not wishing to be bound by theory, it is believed that themorphology and dimension of the Au nanoparticles depend on theconcentrations of the seed particles and CTAB, in addition to thereactants (Au³⁺ and AA). All of the above factors have been found to beinterdependent, thus giving rise to interesting combinations for variousshapes (Table 1). For example, at 1.6×10⁻² M CTAB and 2.0×10⁻⁴ M Au³⁺ions, nanorods, and other particles with triangular and square outlinesare formed, for an AA concentration 1.6 times the Au³⁺ ion concentration(FIG. 1A). On increasing the AA concentration, rod length and yielddecrease, and particles with hexagonal shapes appear (FIG. 1B). Upon afurther increase in AA, cube-shaped particles are formed (FIG. 1C).Simultaneous adjustment of all four reactant concentrations can producemonodisperse Au nanoparticles with hexagonal and cubic profiles in veryhigh yield (˜90%) at room temperature in aqueous solution. FIG. 1D showsthat smaller particles with triangular outlines are the major productinstead of cubic ones, when the seed concentration is raised, keepingother parameters the same as for the cubic shapes.

TABLE 1 [CTAB]/M [Au]_(seed)/M [Au₃₊]/M [AA]/M Shape/ProfileDimension_(§) % Yield 1.6 × 10⁻² 1.25 × 10⁻⁸ 2.0 × 10⁻⁴ 6.0 × 10⁻³ Cube66 nm ~85 1.6 × 10⁻² 1.25 × 10⁻⁸ 2.0 × 10⁻⁴ 3.0 × 10⁻³ Hexagon 70 nm ~801.6 × 10⁻² 1.25 × 10⁻⁷ 2.0 × 10⁻⁴ 6.0 × 10⁻³ Triangle 35 nm ~80 1.6 ×10⁻² 1.25 × 10⁻⁸ 4.0 × 10⁻⁴ 6.4 × 10⁻⁴ Cube_(a) 90 nm ~70 9.5 × 10⁻²1.25 × 10⁻⁷ 4.0 × 10⁻⁴ 6.0 × 10⁻³ Tetrapod_(a) 30 nm ~70 1.6 × 10⁻² 1.25× 10⁻⁸ 4.0 × 10⁻⁴ 1.2 × 10⁻² Star 66 nm ~50 5.0 × 10⁻² 6.25 × 10_(−7b)5.0 × 10⁻⁴ 3.0 × 10⁻³ Tetrapod 293 nm  ~75 9.5 × 10⁻²  2.5 × 10⁻⁷ 4.0 ×10⁻⁴ 6.4 × 10⁻⁴ Branched_(a) 174 nm  ~95 _(§)For triangular profile andcubes, this corresponds to edge lengths; for hexagonal profile, thiscorresponds to the distance between opposite sides; and for tetrapodsand branched particles, this corresponds to center-to-tip distances. Forcubes, triangles and hexagons, the dimensions are averaged over ~120particles and are reproducible to within 5% of the given value; for theother shapes, the dimensions are averaged over ~120 particles and arewithin ~10% of the given value. _(a)6.0 × 10−5M AgNO3 was used._(b)Seed(5) was used here, otherwise results are reported for seed(1).

2. Mechanism and Theory

While not wishing to be bound by theory, it is believed that theformation of various shapes is likely the outcome of the interplaybetween the faceting tendency of the stabilizing agent and the growthkinetics (rate of supply of Au⁰ to the crystallographic planes). Ahmadi,T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science1996, 272, 1924-1925. Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.;Gardea-Torresdey, J. J. Vac. Sci. Technol. B 2001, 19, 1091-1103. Yang,C. Y.; Heinemann, K.; Yacaman, M. J.; Poppa, H. Thin Solid Films 1979,58, 163-168. Renou, A.; Gillet, M. Surf Sci. 1981, 106, 27-34. Petroski,J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998,102, 3316-3320. For example, both fcc cubooctahedral and icosahedralparticles may show hexagonal profiles under TEM. While not wishing to bebound by theory, it is believed that, as in the case of Pt⁰-polymersystems reported by El-Sayed et al., the shape of the fine Au seedsproduced in the presence of CTAB is faceted with the most stable {111}faces solvent-accessible. Petroski, J. M.; Wang, Z. L.; Green, T. C.;El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316-3320. CTAB moleculesappear to bind more strongly to the {100} than the {111} faces. Johnson,C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater.Chem. 2002, 12, 1765-1770. Thus, lower CTAB and higher AA concentrationconditions favor the faster formation and deposition of Au⁰ onto the{111} faces, leading to their disappearance and the formation of {100}faces, thereby producing cubic shapes. Under similar (or slightlyhigher) CTAB concentrations and slightly lower AA concentrationconditions, truncated octahedra with both {100} and {111} faces can beproduced. The formation of truncated fcc shapes has also been previouslyobserved in the presence of passivating agents for gold nanocrystals.Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J. J.Vac. Sci. Technol. B 2001, 19, 1091-1103. A good yield of cube-shapedparticles can be obtained even at low [AA] conditions, such as for a[AA]=1.6×[Au³⁺], if a small quantity of AgNO₃ is added to the system(FIG. 2A). These particles appear to have rough surfaces. The edgelength of these particles is a function of both [Au³⁺] and [seed].However, noncubic shapes form especially upon decreasing or increasingthe concentrations of Au³⁺ ions. If CTAB concentration is increased from1.6×10⁻² to 9.5×10⁻² M, a very high yield (˜97%) of gold particles withrectangular outline to cylindrical rod-shapes are formed, depending onthe concentration ratio of seed particles to Au³⁺ ions (FIGS. 2B, C andD). Preliminary high-resolution TEM data show that the rectangularblocks are single-crystalline in structure. In electroless metalplating, reduced Ag⁺ ions act as sacrificial seeds for the reduction ofAu³⁺ ions to form Au tubes/rods. Menon, V. P.; Martin, C. R. Anal. Chem.1995, 67, 1920-1928. While not wishing to be bound by theory, it isbelieved that this mechanism is not operative in this invention, sincesubstantially no particle formation is detected in the absence of seedsin the present experimental time scale. El-Sayed et al. have proposedthat silver ions could assist in the template elongation by pairing withBr⁻ ions of CTAB. Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15,1957-1965.

A lowering of the ratio of concentrations of seed to Au³⁺ ions alongwith an increase in the concentration of AA can result in the formationof branched Au particles, depending on the concentrations of CTAB andsilver nitrate (FIG. 3). However, in one aspect, silver nitrate is notessential for the branching. The yield of the branched particlesproduced is as high as ˜70%. The four arms in larger tetrapods areclearly out of plane. The release of stress/strain effects in the growthof Au nanoparticles has been previously observed to give rise toanomalous shapes. Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.;Gardea-Torresdey, J. J. Vac. Sci. Technol. B 2001, 19, 1091-1103. Yang,C. Y.; Heinemann, K.; Yacaman, M. J.; Poppa, H. Thin Solid Films 1979,58, 163-168. Renou, A.; Gillet, M. Surf Sci. 1981, 106, 27-34. However,in the case of semiconductor systems, the formation of branchedstructures typically requires first a relatively high supply of monomergrowth units to the seed and then the evolution of branches of variouskinds is determined by a balancing act between the concentration buildupand the competition of ligand/stabilizing molecules for the particlesurface. Peng, X.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.;Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61. Manna, L.;Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122,12700-12706. Peng, X. Adv. Mater. 2003, 15, 459-463. Peng, Z. A.; Peng,X. J. Am. Chem. Soc. 2001, 123, 1389-1395. Very recently, Chen et al.also proposed that forced reduction of gold ions by ascorbic acidthrough the addition of NaOH is key for branching of particles. Chen,S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am.Chem. Soc. 2003, 125, 16186-16187. Vis-NIR absorption spectra of thesamples containing Au nanocrystals of various shapes show cleardifferences in optical absorption properties (FIG. 4).

The present invention, in one aspect, uses a simple solution-basedseed-mediated growth method where one can controllably vary themorphology and dimension of the Au nanocrystals by the manipulation ofthe synthetic parameters. Moreover, these various shapes can be producedin aqueous solution at room temperature and, in one aspect, by utilizingonly one surfactant, CTAB.

In one aspect, the invention relates to a method for preparing acetyltrimethylammonium bromide-capped gold nanoparticle comprising thesteps of providing a seed solution comprising a gold nanoparticle;providing an aqueous growth solution comprising cetyltrimethylammoniumbromide, hydrogen tetrachloroaurate, and ascorbic acid; and adding aquantity of the seed solution to the aqueous growth solution, therebyproducing a cetyltrimethylammonium bromide-capped gold nanoparticlehaving a shape comprising a cube, a block, a tetrapod, a sphere, a rod,a star, or a dogbone.

In one aspect, the nanoparticle has a shape comprising a cube, a block,a tetrapod, a rod with an aspect ratio of at least about 3.2, or adogbone.

In one aspect, the providing a seed solution step comprises the step ofreducing hydrogen tetrachloroaurate with ascorbic acid in the presenceof cetyltrimethylammonium bromide, thereby producing seeds of less thanabout 4 nm. In a further aspect, the providing a seed solution stepcomprises the step of reducing hydrogen tetrachloroaurate with sodiumborohydride in the presence of trisodium citrate, thereby producingseeds of from about 20 nm to about 30 nm.

In one aspect, the aqueous growth mixture further comprises silvernitrate.

3. Cetyltrialkylammonium Halide-Capped Nanoparticles

In one aspect, the invention relates to a cetyltrialkylammoniumhalide-capped nanoparticle. That is, the gold nanoparticle has at leastone cetyltrialkylammonium halide residue associated with the surface ofthe nanoparticle. In a further aspect, the nanoparticle is acetyltrialkylammonium halide-capped gold nanoparticle. In a furtheraspect, the invention relates to a cetyltrimethylammonium bromide-cappedgold nanoparticle. In one aspect, the invention relates to a compositioncomprising nanoparticulate gold and a cetyltrimethylammonium bromideresidue, wherein the composition has a shape comprising a cube, a block,a tetrapod, a sphere, a rod, a star, or a dogbone. In a further aspect,the shape comprises a cube, a block, a tetrapod, a rod with an aspectratio of at least about 3.2, or a dogbone.

C. ENHANCING RAMAN SIGNALS

1. Methods

The methods and compositions of the invention can be used to enhanceRaman signals. In one aspect, the method comprises the steps ofproviding a sample comprising a metal surface, an analyte adhered to thesurface, and a metallic nanoparticle coupled to the surface, wherein thenanoparticle has a plasmon resonance band; exposing the sample toincident energy of an excitation wavelength that overlaps with themetallic nanoparticle plasmon resonance band; and detecting the Ramansignal of the analyte.

By “coupled,” it is meant that the nanoparticle is in relatively closeproximity to the surface and the resulting combination can operate toenhance the vibrational spectral intensity of an analyte adhered to thesurface. In one aspect, the nanoparticle and the surface can form a“sandwich” with the analyte, resulting in enhanced vibrational spectralintensity of the analyte.

By “adhered,” it is meant that the analyte is associated with thesurface. In one aspect, the analyte is chemically bonded to the surfaceby, for example, at least one covalent bond, ionic bond, coordinationbond, or hydrogen bond. In a further aspect, the analyte is attracted tothe surface by, for example, hydrophobic interactions or hydrophilicinteractions. In a further aspect, the analyte is reversibly associatedwith the surface.

In a further aspect, the method comprises the steps of providing asample comprising a metal surface, a functionalized self-assembledmonolayer adhered to the surface, wherein the self-assembled monolayercomprises an analyte, and a cetyltrimethylammonium bromide-cappedmetallic nanoparticle coupled to the surface; exposing the sample toincident energy of an excitation wavelength; and detecting the Ramansignal of the analyte. In one aspect, the nanoparticle has a plasmonresonance band and the excitation wavelength overlaps with the metallicnanoparticle plasmon resonance band. In a further aspect, the surfacecomprises at least one of gold, silver, copper, or silicon or a mixtureor an alloy thereof. In a further aspect, the analyte comprises a thiolmoiety. In a further aspect, the analyte comprises a carboxylic acidmoiety. In a further aspect, the nanoparticle comprises at least one ofgold, silver, or copper or a mixture or an alloy thereof.

Typically, the nanoparticle produced by the methods of the invention hasa shape. In one aspect, the shape comprises a cube, a block, a tetrapod,a sphere, a rod, a star, or a dogbone. In a further aspect, the shapecomprises a cube and the plasmon resonance band comprises a wavelengthmaximum of about 540 nm. In a further aspect, the shape comprises asphere and the plasmon resonance band comprises a wavelength maximum ofabout 520 nm. In a further aspect, the shape comprises a rod with anaspect ratio of from about 3.2 to about 16 and the plasmon resonanceband comprises a longitudinal wavelength maximum of from about 685 nm toabout 1200 nm and a transverse maximum of about 520 nm. In a furtheraspect, the shape comprises a rod with an aspect ratio of greater thanabout 16 and the plasmon resonance band comprises a wavelength maximumof greater than about 1200 nm. In a further aspect, the shape comprisesa tetrapod or a dogbone and the plasmon resonance band comprises awavelength of about 633 nm.

In one aspect, the invention relates to a method for enhancing a Ramansignal comprising the steps of providing a sample comprising a goldsurface, a functionalized self-assembled monolayer adhered to thesurface, wherein the self-assembled monolayer comprises an analyte, anda cetyltrimethylammonium bromide-capped metallic nanoparticle coupled tothe surface; exposing the sample to incident energy of an excitationwavelength that overlaps with the metallic nanoparticle plasmonresonance band; and detecting the Raman signal of the analyte, whereinthe Raman signal has an enhancement factor of from about 10⁷ to about10⁹ relative to the analyte in solution.

a. Analyte

Those skilled in the art will recognize that there is a great deal oflatitude in the composition of an analyte that yields a distinct Ramanspectrum. For example, in some aspects, the analyte is a molecule. Inother aspects, the analyte is not a molecule: it can be a positively ornegatively charged ion (e.g., Na⁺ or CN⁻). If the analyte is a molecule,it can be neutral, positively charged, negatively charged, oramphoteric. The analyte can be a solid, liquid or gas. Non-molecularspecies such as metals, oxides, sulfides, etc. can serve as theRaman-active species. For example, a film of SiO₂ on Au exhibits aunique and identifiable Raman spectrum. Any species or collection ofspecies that gives rise to a unique Raman spectrum, whether solid,liquid, gas, or a combination thereof, can serve as the analyte.Examples easily number in the many millions and include but are notlimited to Hg, dimethylformamide, HCl, H₂O, CN⁻, polypyrrole,hemoglobin, oligonucleotides, charcoal, carbon, sulfur, rust,polyacrylamide, citric acid, and diamond. In the case of diamond, theunique phonon mode of the particle can be used. For hemoglobin, only theporphyrin prosthetic group exhibits significant Raman activity; thus,complex substances can be used as the analyte if only part of themolecular or atomic complexity is present in the Raman spectrum.

The analyte can also be a polymer to which multiple Raman-activemoieties are attached. In one aspect, the polymer can have differentattached moieties yielding different Raman spectra. The polymer backbonedoes not necessarily itself contribute to the acquired Raman spectrum.In one aspect, the polymer can be a linear chain containing amine orammonium groups to which Raman-active entities are attached. In afurther aspect, the polymer can be a dendrimer, a branched polymer witha tightly controlled tree-like structure, with each branch terminatingin a Raman-active species. A suitable dendrimer structure can have fourgenerations of branches terminating in approximately 45 Raman-activeentities.

Typically, the analyte can be any analyte known to those of skill in theart for analysis by Raman spectroscopy. In one aspect, the analytecomprises a thiol moiety. In a further aspect, the analyte comprises acarboxylic acid moiety. In a yet further aspect, the analyte comprises4-mercaptobenzoic acid (4-MBA) or a derivative or salt thereof. In astill further aspect, the analyte comprises a portion of afunctionalized self-assembled monolayer. That is, in one aspect, theanalyte can be used to prepare a self-assembled monolayer on thesurface.

b. Surface

Typically, the surface can be any surface known to those of skill in theart for use in Raman spectroscopy. In a further aspect, the surfacecomprises at least one of gold, silver, copper, or silicon or a mixtureor an alloy thereof. In a yet further aspect, the surface comprisesgold.

While, in one aspect, the analyte can comprise a portion of afunctionalized self-assembled monolayer, in a further aspect, thesurface can be functionalized with, for example, a self-assembledmonolayer. In such an aspect, the self-assembled monolayer can beselected to have an affinity for an analyte, thereby providing analternate mechanism for adhering an analyte to the surface.

c. Nanoparticle

Typically, the nanoparticle can be any nanoparticle known to those ofskill in the art. In one aspect, the nanoparticles are thecetyltrialkylammonium halide-capped nanoparticles. That is, thenanoparticle has at least one cetyltrialkylammonium halide residueassociated with the surface of the nanoparticle. In a further aspect,the nanoparticle comprises a cetyltrialkylammonium bromide-cappedmetallic nanoparticle. In a further aspect, the invention relates to acetyltrimethylammonium bromide-capped gold nanoparticle.

In one aspect, the nanoparticle comprises at least one of gold, silver,or copper or a mixture or an alloy thereof. In a further aspect, thenanoparticle comprises gold.

In one aspect, nanoparticle has a shape comprising a cube, a block, atetrapod, a sphere, a rod, a star, or a dogbone. In a further aspect,the shape comprises a cube and the plasmon resonance band comprises awavelength maximum of about 540 nm. In a further aspect, the shapecomprises a sphere and the plasmon resonance band comprises a wavelengthmaximum of about 520 nm. In a further aspect, the shape comprises a rodwith an aspect ratio of from about 3.2 to about 16 and the plasmonresonance band comprises a longitudinal wavelength maximum of from about685 nm to about 1200 nm and a transverse wavelength maximum of about 520nm. In a further aspect, the shape comprises a rod with an aspect ratioof greater than about 16 and the plasmon resonance band comprises awavelength maximum of greater than about 1200 nm. In a further aspect,the shape comprising a tetrapod or a dogbone and the plasmon resonanceband comprises a wavelength of about 633 nm.

In certain aspects of the invention, the nanoparticles can be randomaggregates of nanoparticles (colloidal nanoparticles). In otherembodiments of the invention, nanoparticles can be cross-linked toproduce particular aggregates of nanoparticles, such as dimers, trimers,tetramers or other aggregates. Certain alternative aspects of theinvention can use heterogeneous mixtures of aggregates of differentsize, while other alternative aspects can use homogenous populations ofnanoparticle aggregates. In certain aspects of the invention, aggregatescontaining a selected number of nanoparticles (dimers, trimers, etc.)can be enriched or purified by known techniques, such asultracentrifugation in sucrose gradient solutions.

d. Excitation Energy

Typically, any source of excitation energy known to those of skill inthe art can be used in connection with the invention. Suitableexcitation sources include a 514.5 nm line argon-ion laser 370 fromSpectraPhysics, Model 166, and a 647.1 nm line of a krypton-ion laser370 (Innova 70, Coherent), a nitrogen laser 370 (Laser Science Inc.) at337 nm and a helium-cadmium laser 370 (Liconox) at 325 nm (U.S. Pat. No.6,174,677), a light emitting diode, an Nd:YLF laser 370, and/or variousions lasers 370 and/or dye lasers 370. The excitation beam 390 can bespectrally purified with a bandpass filter (Corion) and can be focusedon the Raman active substrate 240, 340 using a 6× objective lens(Newport, Model L6X). The objective lens can be used to both excite theanalytes and to collect the Raman signal, by using a holographic beamsplitter (Kaiser Optical Systems, Inc., Model HB 647-26N18) to produce aright-angle geometry for the excitation beam 390 and the emitted Ramansignal. A holographic notch filter (Kaiser Optical Systems, Inc.) can beused to reduce Rayleigh scattered radiation. Alternative Raman detectors380 include an ISA HR-320 spectrograph equipped with a red-enhancedintensified charge-coupled device (RE-ICCD) detection system (PrincetonInstruments). Other types of detectors 380 may be used, such asFourier-transform spectrographs (based on Michaelson interferometers),charged injection devices, photodiode arrays, InGaAs detectors,electron-multiplied CCD, intensified CCD and/or phototransistor arrays.

Typically, the excitation energy is selected so as to overlap with aplasmon band of a nanoparticle of the invention. In one aspect, theexcitation energy is provided as light of an excitation wavelengthincident upon the surface or upon the composition or upon the analyte.In a further aspect, the incident energy is provided by a visible lightwavelength laser. In a yet further aspect, the incident energy isprovided by a HeNe laser.

In one aspect, the excitation comprises visible light. In a furtheraspect, the excitation comprises ultraviolet light. In a further aspect,the excitation comprises infrared light. In one aspect, the excitationcomprises light of a wavelength of from about 400 nm to about 500 nm,from about 500 nm to about 600 nm, from about 700 nm to about 800 nm,from about 800 nm to about 900 nm, from about 900 nm to about 1000 nm,from about 1000 nm to about 1100 nm, from about 1100 nm to about 1200nm, or of greater than about 1200 nm. In one aspect, the excitationwavelength comprises a wavelength of about 633 nm.

e. Enhancement Factor

In one aspect, use of the methods and compositions of the inventionresults in enhancement of a Raman signal. The amount of enhancement canbe referred to as an enhancement factor (EF). The EF can be expressedrelative to an analyte in solution or an analyte on a surface. In oneaspect, the Raman signal has an enhancement factor of from about 10⁷ toabout 10⁹ relative to the analyte in solution.

In a further aspect, the nanoparticle has a shape comprising a cube, ablock, a tetrapod, a rod with an aspect ratio of at least about 3.2, ora dogbone and the Raman signal has an enhancement factor of from about10¹ to about 10² relative to the analyte in a sample comprising aspherical nanoparticle.

Those skilled in the art of Raman spectroscopy are aware that thegeneral concept of inelastic light scattering has many alternativemanifestations that can be used for detection. The basic “normal” Ramanscattering experiment involves detection/measurement of Stokes-shiftedphotons, i.e., those with a lower energy than the incident photons.Anti-Stokes photons-those with energies greater than the incidentphotons-are also generated in a Raman experiment. While the intensity ofanti-Stokes Raman bands is typically low compared to the Stokes bands,they offer one very significant advantage: the lack of interference fromfluorescence, which by definition occurs at lower energies thanexcitation. In embodiments in which the overall SERS intensity issufficiently high, this may be an attractive method for detection.

For molecules whose absorption spectrum overlaps with the laserexcitation wavelength, Raman experiments can be said to be in resonance;both the theory and practice of resonance Raman are well understood.SERS experiments carried out under these circumstances can also bereferred to as SERRS (surface enhanced resonance Raman scattering).SERRS spectra are typically more intense than normal Raman spectra, andmay provide an additional benefit. Organic molecules that possess highextinctions in the visible region of the spectrum also exhibitrelatively complex molecular structures, and as such might not beoptimal choices for the intermediate layer. On the other hand,coordination complexes can have reasonably high absorptivity and stillpossess simple structures. For example, simple homoleptic complexes ofCu(I) and Cu(II) are often intensely colored (e.g., [Cu(NH₃)₄]⁻).

In addition to SERS and SERRS, there are a variety of other detectionmechanisms contemplated by the instant invention, including but notlimited to surface enhanced infrared absorption spectroscopy (SEIRA),surface enhanced hyperRaman spectroscopy (SEHRS), and its resonantanalog, SEHRRS. In SEHRS and SEHRRS, two photons of frequency A generatea scattering event at a frequency of 2 A. The primary benefit of thismethod is the total lack of interference by fluorescence or any otherbackground process: one can excite a particle with 800 nm light andobserve photons Raman-shifted from 400 nm. In general, for a givenanalyte with N atoms, there are either 3N-5 or 3N-6 unique vibrations;all of these vibrations can be found in either the Raman, hyperRaman, orinfrared spectrum. Indeed, in some aspects, identification can rest on acombination of optical interrogation methods, including methods thatrely on inelastic scattering of photons (e.g., SERS, SERRS, SEHRS, andSEHRRS, in both Stokes and anti-Stokes modes), methods that rely onelastic scattering of photons (e.g., Raleigh scattering and hyperRaleighscattering for particles with dimensions at least 1/10th of theexcitation wavelength), and methods that rely on adsorption, e.g.,SEIRA.

2. Compositions

Typically, various compositions of the invention can be used inconnection with the methods of the invention. In one aspect, theinvention relates to a composition comprising a metal surface, afunctionalized self-assembled monolayer adhered to the surface, whereinthe self-assembled monolayer comprises an analyte, and acetyltrialkylammonium halide-capped metallic nanoparticle coupled to thesurface. In one aspect, the surface comprises at least one of gold,silver, copper, or silicon or a mixture or an alloy thereof. In afurther aspect, the nanoparticle comprises a cetyltrialkylammoniumbromide-capped metallic nanoparticle. In a further aspect, thenanoparticle comprises a cetyltrimethylammonium bromide-capped metallicnanoparticle.

D. EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompounds, compositions, articles, devices and/or methods claimed hereinare made and evaluated, and are intended to be purely exemplary of theinvention and are not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to ensure accuracywith respect to numbers (e.g., amounts, temperature, etc.), but someerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

1. Preparation of Shaped Nanoparticles

a. General Procedures

In a typical seed synthesis, a 10 mL solution of Au seeds is prepared bythe reduction of HAuCl₄.3H₂O (2.5×10-4 M) by ice-cold NaBH₄ (6.0×10-4 M)in the presence of cetyltrimethylammonium bromide (CTAB, 7.5×10-2 M).The NaBH₄ solution is added at a time to the solution containing CTABand HAuCl₄ and the reaction mixture is then shaken (or magneticallystirred) for two minutes allowing the escape of the gas formed duringthe reaction. These seeds are likely four nm or smaller in diameter andare designated as seed(1).

Au seeds are produced at two other CTAB concentration conditions also,namely 9.5×10⁻²M and 5.0×10⁻² M. One can produce various shapes usingthese two seeds as well. However, the amounts of seed particles and AArequired for a given Au³⁺ ion and CTAB concentration are different foreach of them.

The same is true for a seed sample produced from 1.0×10⁻⁴ M HAuCl₄solution. Relatively large seeds (20-30 nm in diameter) were alsoprepared by the reduction of the same quantity of HAuCl₄ (2.5×10⁻⁴M) byascorbic acid (4.0×10⁻⁴ M) in the presence of tri-sodium citrate(2.5×10⁻⁴ M) as stabilizer and designated these seeds as seed (5).Cylindrical rod-shaped and armed Au particles were produced in highyield by using these larger seed particles. All the seeds' are usedbetween 2 and 24 hours after their preparation.

In a typical growth reaction, 0.20 mL HAuCl₄ solution is added to 4.75mL CTAB solution (0.10 M) followed by the addition of 0.03 mL AgNO₃(0.01 M), 0.032 mL L-ascorbic acid (0.10 M), and 0.01 mL Au seedsolutions. The solution is gently mixed by inversion of the test tubeafter the addition of every component.

b. TEM Studies

TEM images are obtained by Hitachi H-8000 or JEOL JEM-100CXII electronmicroscope. Typically 1.5 mL of the solution is centrifuged for 10 minat a speed of 10000 rpm to precipitate the solid. The supernatant isdiscarded. Then, the solid residue is redispersed in 1.5 mL DI water andcentrifuged again. Finally the solid residue is redispersed in asuitable volume of DI water depending on the quantity of the residue. 7μL of this solution is dropcast on a TEM grid and allowed to dry in openatmosphere.

c. Role of Silver Nitrate

Rods of cylindrical shape and penta-twinned structure and higher lengthalong with a substantial number of spherical particles are formed whensilver nitrate is not used in the system. Preliminary studies show thatsingle crystalline rods are formed in the presence of silver nitrate. Inthe presence of silver nitrate, transverse growth typically occurs moreand the seeds hardly form spherical particles. EDAX studies show that 3to 7 wt % of silver is associated with the particles. It can be presentas alloyed Ag⁰ in the particle or as adsorbed Ag⁺ on the particlesurface.

2. Surface Enhanced Raman Spectroscopy

A. NANOSHAPE SYNTHESIS

Gold nanoparticles including cubes (edge length=61±3 nm), blocks (aspectratio 2.4±0.4, length=81±9 nm, width=34±3 nm), tetrapods (centerwidth=81±18 nm, edge length=107±18 nm), spheres (diameter=29±6 nm), androds, aspect ratio 3.2±0.6 (length=55±7 nm, width=17±3 nm), 4.4±0.9(length=62±6 nm, width=14±3 nm), and 16.0±5.3 (length=372±119 nm,width=23±4 nm), are prepared using the above procedure. “Dogbone”-shapedgold nanoparticles (center width=21±2 nm, end width=30±4 nm,length=68±11 nm) are prepared by adding 10 mL of as-prepared goldnanorods (aspect ratio 4.4) to a growth solution containing 8.5 mL of0.1 M CTAB, 0.5 mL of 0.01 M HAuCl₄, and 1.0 mL of 0.1 M ascorbic acid.The unstirred solution changes color from tan to dark blue in ˜2 minutesand is stored at room temperature for >2 hours prior to use.

B. NANOPARTICLE IMMOBILIZATION OF SELF-ASSEMBLED MONOLAYERS

Gold substrates are prepared by sputtering 10 nm of chromium, followedby 100 nm of gold, on piranha-cleaned glass microscope slides. 4-MBAself-assembled monolayers (SAMs) are formed by immersing the gold-coatedglass slides, cut to 1 cm², into a 1 mM ethanolic solution of 4-MBA for24 hrs, rinsed thoroughly with ethanol, and dried with nitrogen.CTAB-protected nanoparticles are immobilized onto 4-MBA SAMs using asimilar protocol described previously for the immobilization of goldnanorods on 16-mercaptohexadecanoic acid (16-MHA) SAMs. See Gole, A.;Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117-7122. Briefly,10 mL of as-prepared nanoparticles are centrifuged at various speedsdepending on nanoparticle size (7,000-14,000 RPM) and the pellet isredispersed in 1 mL of deionized water to remove excess surfactant inthe supernatant. After two centrifugation steps, 1 mL of nanoparticlesolution is diluted to 3 mL with deionized water. The 4-MBA SAMs ongold-coated glass substrates are immersed in the resulting aqueousnanoparticle solutions for 3 hours. Substrates are rinsed with deionizedwater and dried with nitrogen. Under these conditions, the 4-MBA SAM isdeprotonated, allowing for electrostatic binding of the cationicCTAB-capped gold nanocrystals.

C. INSTRUMENTATION

Surface enhanced Raman spectra are collected using a Detection LimitSolution 633 Raman system using a 633 nm helium neon laser with 25 mWlaser power at the sample. Integration times are given in the figurecaptions for each spectrum. Spectra are corrected for background usingGRAMS 32 software (Galactic). Absorption spectra are acquired using aCARY 500 Scan UV-vis-NIR spectrometer. Scanning electron micrographs areacquired using an FEI Quanta 200 environmental scanning electronmicroscope. Various magnifications are used and are provided in thefigure captions. Raman spectra and SEM images are acquired on the samesample in approximately the same region in order to minimize possibleeffects from sample heterogeneity. Transmission electron microscopy isperformed on either a Hitachi H-8000 or a JEOL 100CXII instrument.

D. RESULTS

Nanoparticle plasmon resonance and immobilization. FIG. 5 showsabsorption spectra of gold nanoparticles of various shapes, includingrods, cubes, dogbones, tetrapods, and blocks. Cubes and spheres haveonly one plasmon band at ˜540 and ˜520 nm, respectively. Rod-shapednanoparticles have plasmon bands corresponding to both transverse andlongitudinal absorption, where the wavelength of longitudinal absorptionincreases with aspect ratio, from 685 nm for aspect ratio 3.2 rodsto >1200 nm for aspect ratio 16 rods. Dogbones, tetrapods, and blockshave multiple plasmon bands from 525 to 775 nm corresponding to thevariable dimensions along multiple axes of these particles. Aspect ratio3.2 nanorods, tetrapods, and dogbones have significant plasmonabsorption overlap with the HeNe laser excitation source used in ourSERS experiments (632.8 nm). Resonance between the incident radiationand the electronic absorption maxima should contribute to greater SERSenhancement for these nanoparticles than those without appreciableabsorption at 632.8 nm. Other chemical effects of these nanoparticlescan contribute to large SERS enhancements, including the surface freeenergy of the nanocrystals and radius of curvature of the nanoparticlefeatures (i.e., lightning rod effect).

The utility of both SEM and AEM has been demonstrated in imaging aspectratio 18 nanorods immobilized on 16-MHA monolayers. Gole, A.; Orendorff,C. J.; Murphy, C. J. Langmuir 2004, 20, 7117-7122. For nanoparticles<100 nm in size and in various of shapes, AFM can be an ideal techniquefor imaging them on surfaces, as its lateral resolution is superior tothat of SEM. However, imaging these surfaces in contact or in tappingmode AFM typically perturbs the monolayer-nanoparticle architecture bymoving nanoparticles on the surface with the AFM tip. While not wishingto be bound by theory, it is believed that this is likely a consequenceof the smaller particle sizes employed herein; there is less surfacearea in contact with the SAM, leading to fewer favorable electrostaticinteractions between nanoparticles and the underlying substrate.Therefore, scanning electron micrographs can be acquired for immobilizednanoparticles in order to calculate nanoparticle density.

FIG. 6 shows representative SEM images of nanorods (aspect ratio 3.2,4.4, and 16), blocks, tetrapods, dogbones, cubes and spheres immobilizedon 4-MBA SAMs. Since the size of nanoparticles >100 nm, with theexception of aspect ratio 16 rods, and near the practical resolution ofthe instrument (˜10-20 nm), it can be difficult to distinguish differentshapes of nanoparticles. Therefore, TEM images of these nanoparticlesare provided as insets in FIG. 6. In FIG. 6 a, aspect ratio 16 nanorodsare uniformly distributed on the surface and are generally isolated,with a density of 17 rods/μm². This is a slightly higher density ofnanorods than reported previously for 16-hexadecanoic acid SAMs. Gole,A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117-7122. Thesmaller nanoparticles are well dispersed on the 4-MBA surface and numberdensities are calculated to be ˜44 rods/μm² for aspect ratio 3.2nanorods, ˜38 rods/m² for aspect ratio 4.4 nanorods, ˜9.5 blocks/μm²,and 11 tetrapods/μm², 9 cubes/μm², ˜32 spheres/μm², and ˜24dogbones/μm². The number density of nanoparticles is a factor indetermining not only surface enhancement factors (EF), but also incomparing the SERS spectra of 4-MBA with different immobilized shapes.

SERS of 4-MBA SAMs using different shaped gold nanoparticles. A schemeof this sandwich geometry for acquiring SERS spectra of SAMs is shown inFIG. 7. In this architecture, the analyte molecules are SAM molecules,and are sandwiched between the smooth gold substrate and theelectrostatically-immobilized gold nanocrystals. Raman spectra of 0.01 M4-MBA alone, a 4-MBA SAM on a gold substrate with no nanoparticles, and4-MBA SAMs on gold in the sandwich geometry with different immobilizednanocrystals are shown in FIG. 8. The corresponding peak frequencyassignments are provided in Table 2. Raman spectra and SERS spectra of4-MBA are comparable to those reported previously for 4-MBA in aqueoussolution and adsorbed onto gold substrates. Michota, A.; Bukowska, J. J.Raman Spectrosc. 2003, 34, 21-25. Park, H.; Lee, S. B.; Kim, K.; Kim, M.S. J. Phys. Chem. 1990, 94, 7576-7580. Lin-Vien, J. G.; Golthup, M. B.;Fateley, W. G. Grasselli, J. G. The Handbook of Infrared and RamanCharacteristic Frequencies of Organic Molecules; Academic Press: NewYork, 1991. Characteristic vibrational modes including ν(CC)ring-breathing modes (1070 and 1575 cm⁻¹) observed in the Raman spectraof aqueous 4-MBA and SERS spectra of 4-MBA, while other less intensemodes including δ (CH) (1132 and 1173 cm⁻¹) and ν_(s)(COO⁻) (1375 cm⁻¹)are observed in SERS spectra with immobilized nanoparticles, but arebelow the signal-to-noise for the Raman spectra of 0.01 M aqueous 4-MBA.

TABLE 2 Peak Frequency (cm⁻¹) 4-MBA SAM on gold 0.01M 4-MBAAssignment^(a,b) 1070 1073 ν(CC)_(ring) 1132 ^(c) δ(CH) 1173 ^(c) δ(CH)1357 ^(c) ν_(s)(COO⁻) 1575 1575 ν(CC)_(ring) ^(a)Assignments fromreferences 42-44. ^(b)δ = bend or deformation; ν = stretch; _(ring)=ring breathing mode; ^(a)= antisymmetric. ^(c)Not Observed

Raman spectra of aromatic thiol SAMs on smooth gold substrates has beenreported previously. Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.;Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212-1221. However, theinstrument used in these experiments was limited to acquiring onlysingle spectral integrations, opposed to multiple accumulations toimprove the signal-to-noise ratio. No characteristic 4-MBA vibrationalmodes are observed in the spectra of 4-MBA SAMs on gold substrates inthe absence of immobilized nanoparticles. These observations are in goodagreement with those obtained by Zheng et al. for SERS of 4-ATP SAMs onsilver substrates with adsorbed silver nanoparticles in the samesandwich geometry. Zheng, J.; Zhou, Y.; Li, X.; Ji, Y.; Lu, T.; Gu, RLangmuir 2003, 19, 632-636. In that work, characteristic vibrationalbands for 4-ATP SAMs on polished silver substrates were readily observedafter the immobilization of silver colloids, but were not observed forthe SAMs in the absence of the colloids. Zheng, J.; Zhou, Y.; Li, X.;Ji, Y.; Lu, T.; Gu, R Langmuir 2003, 19, 632-636.

Surface enhancement factors (EF) are calculated for each of thedifferent nanoparticle shapes using the following expression:

EF=[I _(SER)]/i[I_(Raman) ]×[M _(Bulk)]/[M_(Ads)]  Eq. 1

where M_(Bulk) is the number of molecules sampled in the bulk, M_(Ads)is the number of molecules adsorbed and sampled on the SERS-activesubstrate, I_(SERS) is the intensity of a vibrational mode in thesurface-enhanced spectrum, and I_(Raman) is the intensity of the samemode in the Raman spectrum. For all spectra, the intensity of the ν(C—C)ring-breathing mode (˜1070 cm⁻¹) is used to calculate EF values.Ideally, the Raman spectra of the 4-MBA SAM would be used to normalizeSERS spectra in determining EF values as described by Taylor et al.Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H.Appl. Spectrosc. 1999, 53, 1212-1221. However, as described above,instrumental limitation prevent the acquisition of interpretable Ramanspectra of the 4-MBA SAM in the absence of immobilized nanoparticles.Therefore, the spectrum of aqueous 0.01 M 4-MBA is used to normalize theSERS data in the EF calculation. All spectra are normalized foracquisition time. The number of molecules sampled in the SERSexperiments is determined by calculating the total two-dimensional areaor “SERS footprint” occupied by the nanoparticles in the illuminatedlaser spot on the surface. This is approximated by multiplying thenumber density of nanoparticles (from the SEM images in FIG. 6), theilluminated spot size (˜0.2 mm dia. at the focal point), and thenanoparticle footprint area (from the TEM images in FIG. 6) to give thetotal SERS surface area sampled. This number is multiplied by thebonding density of 4-MBA molecules in a SAM, ˜0.5 nmol/cm² to give thetotal number of molecules sampled in the SERS experiments. Taylor, C.E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl.Spectrosc. 1999, 53, 1212-1221. It is noteworthy that there are manymore molecules in the SAM that are not sandwiched between nanoparticlesand the gold substrate. Taylor et al. determined the enhancement factorof smooth, vapor deposited gold substrates to be ˜2 at 514.5 nm and ˜64at 720 nm. Since EF values expected for these nanoparticle-SAM samplesshould be several orders of magnitude greater than a factor of 64, SERScontributions from SAM molecules not sandwiched between nanoparticlesand the gold substrate are assumed to be negligible.

Calculated EF values for 4-MBA SAMS with immobilized nanocrystals aregiven in Table 3.

TABLE 3 Nanoparticle shape EF Spheres 1.62 ± 0.63 × 10⁷ Aspect ratio 3.2rods 1.02 ± 0.40 × 10⁸ Aspect ratio 4.4 rods 1.04 ± 0.11 × 10⁸ Aspectratio 16 rods 1.08 ± 0.08 × 10⁸ Tetrapods 7.16 ± 0.09 × 10⁸ Dogbones1.61 ± 0.11 × 10⁹ Cubes 2.43 ± 0.21 × 10⁹ Blocks 2.65 ± 0.19 × 10⁹

All EF values are between 10⁷ and 10⁹, which is significantly greaterthan those estimated for aromatic SAMs on rough planar substrates(˜10⁶). Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M.H. Appl. Spectrosc. 1999, 53, 1212-1221. VanDuyne, R. P.; Hulteen, J.C.; Triechel, D. A. J. Chem. Phys. 1993, 99, 2101-2115. Likewise, theseEF values are also greater than those estimated for 2-aminothiophenoladsorbed to unaggregated gold nanorods (˜10⁵) and 2,4-dinitrotolueneadsorbed on silver nanowires (˜10⁵). Nikooballht, B. Wang, J. El-Sayed,M. A. Chem. Phys. Lett. 2002, 366, 17-23. Tao, A.; Kim, F.; Hess, C.;Goldberger, J.; He, R.; Sun, Y.; Xia, Y, Yang, P. Nano Lett. 2003, 3,1229-1323. This indicates that plasmon coupling between the nanocrystals(LSP) and the gold substrate surface (SPP) contributes to significantlocalized field enhancement for 4-MBA molecules in the goldnanoparticle-planar substrate sandwich, resulting in large SERSintensities. LSP-SPP coupling is believed to contribute to increase SERSintensities observed by Zhang et al. for 4-ATP SAMs on silver withimmobilized silver colloids. Lyon, L. A.; Pena, D. J.; Natan, M. J. J.Phys. Chem. B 1999, 103, 5826-5831. However, no EF values were estimatedin that work for comparison. These SERS results are comparable to thoseobtained by Natan and others for SPR colloid-enhanced sandwich assays,where LSP-SPP interactions contribute to SPR angle shift enhancement.Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70,5177-5183.

While not wishing to be bound by theory, it is believed that, for thesenanoparticles, enhancements are a combination of plasmon absorption, orEM contributions, and chemical effects. Cubes, blocks, and dogbones havethe largest surface enhancement factors, ˜10⁹, and spheres have thesmallest enhancement, 10⁷. However, cubes, blocks, and dogbones haveless absorption at 633 nm than tetrapods and aspect ratio 3.2 nanorods,but still give greater Raman enhancement by a factor of 10. Thisindicates that differences in enhancements observed for each of thesenanoparticle shapes is less dependent on resonance with the incidentradiation source (EM factors) than other chemical effects.

One contributing chemical effect is the absorption strength of 4-MBA ofthe gold nanoparticles. El-Sayed and coworkers determined that greaterRaman scattering enhancement is observed for molecules on Au {110} thanAu{111}, because the Au{110} has a higher surface energy. Nikoobakht, B.Wang, J. El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17-23. However, inthis case the SERS analyte is immobilized in a self-assembled monolayer,and the gold nanocrystals are capped with CTAB. Therefore, chemicalenhancement effects due to preferential binding of the SERS analyte todifferent crystal faces of the gold colloidal particles is not likely.While not wishing to be bound by theory, it is believed that thedifferences in EF values between nanoparticle shapes is due to thelightning rod effect. Schatz, G. C. Acc. Chem. Res. 1984, 17, 370-376.Gersten, J. I., J. Chem. Phys. 1980, 72, 5779-5780. In general, greaterfield enhancements are observed near the sharpest surface features.Dogbones, tetrapods, cubes, and blocks all have more well-defined edges,corners and have generally sharper surface features than rods andspheres, shown in FIG. 6. As observed here, the lightning rod effectresults in greater localized field enhancement for dogbones, cubes,tetrapods, and blocks than rods, and rods having greater fieldenhancement than spheres.

E. CONCLUSIONS

Gold nanoparticles of various shapes and sizes are immobilized on 4-MBASAMs on gold via electrostatic interactions, as described previously.Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117-7122.These nanoparticle-planar substrate sandwich structures are used as SERSsubstrates. No vibrational bands of 4-MBA are observed in the Ramanspectra of the 4-MBA SAM on sputtered gold, but vibrational bands arereadily observed when gold nanoparticles of any shape are immobilized onthe SAM. Results suggest that SERS of the 4-MBA SAMs in this sandwichgeometry originates from plasmon coupling between localized surfaceplasmon of the nanoparticles and surface plasmon of the gold substrate;creating a large localized electromagnetic field enhancement, or SERS“hot spot,” for the 4-MBA molecules between the nanoparticles and theplanar substrate. Differences in surface enhancement are also observedfor nanoparticles of different shape. Results indicate that resonancebetween the incident radiation and the LSP of nanoparticles, EMenhancement, is not sufficient to adequately describe differencesbetween EF values of the various nanoparticle shapes. Chemicalcontributions to SERS including the surface structure and sharpness ofstructural features of the gold nanocrystals contribute to enhanced EFvalues for different shaped nanoparticles.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method for preparing a cetyltrimethylammonium bromide-capped goldnanoparticle comprising the steps of: a. providing a seed solutioncomprising a gold nanoparticle; b. providing an aqueous growth solutioncomprising: i. cetyltrimethylammonium bromide, ii. hydrogentetrachloroaurate, and iii. ascorbic acid; and c. adding a quantity ofthe seed solution to the aqueous growth solution, thereby producing acetyltrimethylammonium bromide-capped gold nanoparticle having a shapecomprising a cube, a block, a tetrapod, a sphere, a rod, a star, or adogbone.
 2. The method of claim 1, wherein the nanoparticle has a shapecomprising a cube, a block, a tetrapod, a rod with an aspect ratio of atleast about 3.2, or a dogbone.
 3. The method of claim 1, wherein theproviding a seed solution step comprises the step of reducing hydrogentetrachloroaurate with ascorbic acid in the presence ofcetyltrimethylammonium bromide, thereby producing seeds of less thanabout 4 nm.
 4. The method of claim 1, wherein the providing a seedsolution step comprises the step of reducing hydrogen tetrachloroauratewith sodium borohydride in the presence of trisodium citrate, therebyproducing seeds of from about 20 nm to about 30 nm.
 5. The method ofclaim 1, wherein the aqueous growth mixture further comprises silvernitrate.